TL;DR: Keep a second usage-metering supplier warm by sending it a small, bounded stream of synthetic and replay-safe events, then reconcile both suppliers against an internal ledger. Put a hard spend ceiling on the exercise and define the amount of traffic you will refuse when that ceiling is reached. A fallback that accepts a health check but cannot reproduce invoice totals is not warm. For a media account platform, the bill is made of metered events retained, processed, and queried across t...